A liquid crystal display device has been adopted widely as a display device for a computer, a mobile phone, a PDA, and a game machine due to the features such as power saving, thinness, and light weight. Generally, the liquid crystal display device includes a liquid crystal display panel and a backlight illuminating the liquid crystal display panel from a back surface thereof. The liquid crystal display panel has a configuration in which a liquid crystal layer is interposed between an active matrix substrate and a counter substrate.
The active matrix substrate includes a glass substrate on which a plurality of pixels including TFTs and pixel electrodes are formed. A plurality of the pixels are arranged in a matrix. A counter substrate includes a glass substrate on which a counter electrode and a color filter are formed. The color filter has coloring layers corresponding the respective pixels.
Due to such a configuration, in the liquid crystal display device, a voltage applied between each pixel electrode and the counter electrode is adjusted, whereby the transmittance of the liquid crystal layer is adjusted for each pixel. Then, an image is displayed on a display screen by illumination light of the backlight, transmitted through the liquid crystal layer.
In recent years, the liquid crystal display device may have a function of capturing an image, in addition to the function of displaying an image (see, for example, JP 2006-3857 A). In the liquid crystal display device disclosed by JP 2006-3857 A, a liquid crystal display panel has a plurality of photodiodes and peripheral circuits for each pixel on an active matrix substrate.
The photodiode outputs an electric signal in accordance with the amount of incident light. The peripheral circuit is provided for each photodiode and has a capacitance. Each peripheral circuit, for example, allows charge to be stored in the capacitance in accordance with the level of an electric signal output by the photodiode, and generates a photodetective signal whose voltage level varies depending upon the amount of the stored charge. Alternatively, each peripheral circuit causes the capacitance to discharge the stored charge in accordance with the level of an electric signal output by the photodiode, and generates a photodetective signal whose voltage level varies depending upon the amount of charge remaining in the capacitance. Furthermore, each peripheral circuit outputs the photodetective signal to a detection circuit provided outside of a display area through a reading line extending in a direction perpendicular to the screen.
The detection circuit collects signals output from the respective peripheral circuits to generate image data, subjects the image data to image processing, and outputs the resultant image data outside. Thus, in the liquid crystal display device disclosed by 2006-3857 A, the image of an object present on an observer side of the display surface of the liquid crystal display panel can be captured.